1. Field of the Invention
The present invention relates to a communication method, a communication apparatus to which the communication method is applied, a reception method and a reception apparatus therefor which are suitable for application to a radio telephone system, for example.
2. Description of the Related Art
In a mobile communication such as a radio telephone system or the like, a multiple access in which a plurality of mobile stations (terminal apparatus or subscribers) are permitted to access a single base station is employed. In this case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access system (FDMA: Frequency Division Multiple Access), a time division multiple access system (TDMA: Time Division Multiple Access), a code division multiple access system (CDMA: Code Division Multiple Access) and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical carrier wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a reception side takes code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spread spectrum, and transmits signals to respective mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station discriminates each of the mobile stations by different spread codes to one another.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in secrecy of telephone conversation. Therefore, the system is suitable for a radio transmission utilizing mobile stations such as a portable telephone apparatus and so on.
In the CDMA system, it is difficult to establish a precise communication relationship between mobile stations. Therefore, each communication between respective mobile stations cannot be dealt completely separately, and hence another mobile station can become a source of interference upon communication with a mobile station. Further, data is spread within a particular frequency band in this system. Therefore, it is necessary to define a band width in advance over which the data is spread (i.e., a band width for use of transmission). Therefore, it is difficult to change the transmission band width.
The above matter will be described more concretely. FIGS. 1A and 1B shows a model in which a transmission signal of a particular user is extracted by an inverse spread from transmission signals of stations (users) which are spectrum spread with predetermined codes and multiplexed, for example. As shown in FIG. 1A, if a signal of a user U0 is to be extracted by the inverse spread from signals of eight users U0 to U7 which are multiplexed with codes, then as shown in FIG. 1B, the signal of the user U0 can actually be extracted. However, signals of other users U1 to U7 which are dealt by the same base station also become an interference source, serving as a noise. This fact results in deterioration of an S/N characteristic. For this reason, in a radio transmission employing the CDMA system, electric wave does not reach well due to the deterioration caused by the interference, which in fact narrows a service area. Further, interference due to other users can be suppressed only by an amount of inverse spread gain which is obtained in a process of spectrum inverse spread. Therefore, a number of users (mobile stations) permitted to access is limited and a channel capacity becomes small.
There can be considered a communication system for solving such problems in which a multicarrier signal having a plurality of subcarriers is transmitted as a transmission signal of one system. However, since in a radio telephone system it is necessary for the base station to simultaneously access a plurality of terminal apparatus, if such transmission system using such multicarrier signal is applied thereto, then the base station requires complicated transmission processing systems each for converting a data into a multicarrier signal to then carry out a transmission processing as much as the terminal apparatus which the base station simultaneously access, and also requires complicated reception processing systems each for converting a multicarrier signal subjected to a reception processing into an original data as much as the terminal apparatus which the base station simultaneously access. Hence, an arrangement of the transmission and reception system of the base station disadvantageously becomes complicated.